Die cushion system for press machine

ABSTRACT

The present invention relates to a die cushion system for press machine where elevation of a lower press die is controlled in relation to the elevation of an upper press slide in a press machine, the die cushion system for press machine comprising: a hydraulic cylinder supporting and driving a press die; a proportional flow control valve supplying working fluid to a lower port of the hydraulic cylinder to raise it and receiving the working fluid within the hydraulic cylinder for discharging it while the hydraulic cylinder is being lowered; a constant pressure supplying means supplying pressured fluid to an upper port of the hydraulic cylinder to lower the hydraulic cylinder; and a controller controlling operation of the proportional flow control valve to adjust lowering and raising of the press die according to the position of the press slide through a slide position detector and a die cushion position detector, and during the lowering process of the press slide, the controller enabling the press die to exert multi-level reacting force onto the press slide. Accordingly, response speed of the die cushion in relation to the press slide is enhanced to exert forming pressure of various patterns so that elastic restoration and residual stress due to a plastic deformaton of a subject of press forming can be minimized.

TECHNICAL FIELD

The pesent invention relates to a die cushion system for press machine where elevation of a lower press die is controlled in relation to the elevation of an upper press slide in a press machine which performs punching, drawing and etc. of a metal plate.

BACKGROUND ART

A conventional die cushion system for press machine, as can be seen in a Korean Patent Application Publication No. 2007-0085303 (publication date: Aug. 27, 2007), has one flow control valve and one flow supply pump to achieve an elevation, that is going up and down, of a die cushion. However, response speed in such a system is slow, so that forming pressure of only a comparably simple pattern can be achieved where a press slide receives pressure changing from high to low level only (refer to FIG. 3 of the Korean Patent Application Publication).

However, there may be elastic restoration and residual stress in the metal plate which has been formed with the forming pressure of such a simple pattern, which causes defects in the formed product.

Especially, the conventional die cushion system which exerts forming pressure of such a merely simple pattern may cause tearing of the formed product in operations which require a considerable change in the plastic deformation during the press forming such as deep drawing of a high strength steel sheet.

DISCLOSURE Technical Problem

Accordingly, it is object of the present invention to provide a die cushion system for press machine which can enhance response speed of a die cushion in relation to a press slide to exert forming pressure of various patterns so that elastic restoration and residual stress due to a plastic deformaton of a subject of press forming can be minimized.

Technical Solution

In order to accomplish the above object, the present invention provides a die cushion system for press machine comprising: a hydraulic cylinder supporting and driving a press die disposed at a lower side in relation to a press slide driven to be raised and lowered at an upper side; a proportional flow control valve receiving working fluid from a tank via a first pump and supplying it to a lower port of the hydraulic cylinder to raise the hydraulic cylinder, and receiving the working fluid within the hydraulic cylinder from the lower port and discharging it to the tank while the hydraulic cylinder is being lowered; a constant pressure supplying means receiving pressured fluid from the tank via a second pump and supplying it to an upper port of the hydraulic cylinder to lower the hydraulic cylinder with the pressured fluid; a slide position detector for detecting position of the press slide; a die cushion position detector for detecting position of the press die; and a controller controlling operation of the proportional flow control valve to adjust lowering and raising of the press die according to the position of the press slide through the slide position detector and the die cushion position detector, and during the lowering process of the press slide, the controller controlling the operation of the proportional flow control valve so that the press die can exert multi-level reacting force onto the press slide.

According to an aspect of the present invention, the multi-level reacting force is controlled to generate an initial load at the time the press slide is lowered and starts contacting the press die, a maximum load greater than the initial load and a final load smaller than the initial load.

According to an aspect of the present invention, a first interim load smaller than the initial load is generated between the initial load and the maximum load, and a second interim load smaller than the final load is generated between the maximum load and the final load.

According to an aspect of the present invention, at least one of the first interim load and the second interim load is 0 (zero) load.

According to an aspect of the present invention, the press slide and the press die are separated from each other during the 0 (zero) load.

According to an aspect of the present invention, the controller controls the operation of the proportional flow control valve for the press die to be lowered at a speed lower than that of the press slide so as to relieve contacting impact at the time the press slide is lowered and start to contact the press die.

Advantageous Effects

According to the present invention as described above, the die cushion system for press machine further comprises a constant pressure supplying means for lowering a hydraulic cylinder in addition to a proportional flow control valve for raising the hydraulic cylinder to enhance response speed of a press die.

Therefore, multi-level reaction force can be applied to a lowering press slide so that multi-level stress control can be applied to a metal plate being formed to remove elastic restoration and residual stress after the forming process.

Also, the response speed of the press die can be enhanced and the multi-level reaction force can be achieved so that forming pressure of various patterns can be exerted. Therefore, products of high quality can be formed even in operations which require a considerable change in the plastic deformation such as deep drawing.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a die cushion system for press machine according to an exemplary embodiment of the present invention;

FIG. 2 to FIG. 6 show graphs of position control and load control of a press die in a die cushion system for press machine that can be realized by the schematic diagram of FIG. 1.

BEST MODE

As shown in FIG. 1, a die cushion system for press machine 10 according to an exemplary embodiment of the present invention comprises a hydraulic cylinder 11 for elevating a lower press die 2 in relation to an upper press slide 1, a proportional flow control valve 12 for raising the hydraulic cylinder 11, a constant pressure supplying means 13, 14 for lowering the hydraulic cylinder 11, and a controller 15 for controlling elevation of the press die 2 in relation to the elevaton of the press slide 1.

The proportional flow control valve 12 receives working fluid of a predetermined pressure, for example 130 to 150 [bar], from a tank 16 via a pump 17 and supplies it to a lower port 11 a of the hydraulic cylinder 11 via a working fluid supplying line 18 to raise the hydraulic cylinder 11. Also, the proportional flow control valve 12 performs function of discharging the working fluid ejecting from the lower port 11 a to a drain 16-1 while the hydraulic cylinder is being lowered.

The constant pressure supplying means 13, 14 receives pressured fluid of a predetermined pressure, for example 210 [bar], from the tank 16 via a pump 14 and supplies it to an upper port 11 b of the hydraulic cylinder 11 via a pressured fluid supplying line 13 to lower the hydraulic cylinder 11.

Accordingly, a spool 12 a of the proportional flow control valve 12 moves forward (to the right) to raise the hydraulic cylinder 11 if there is an input of a control signal from the controller 15. Although a lower side pressure P1 is lower than an upper side pressure P2, the hydraulic cylinder 11 drives upwardly because of the difference in the pressure area on a piston 11 c. In this case, the pressured fluid ejecting from the upper port 11 b is temporarily stored in a pressure accumulator 19 via the pressured fluid supplying line 13. After the pressure accumulator 19 is filled, the presured fluid is discharged to the drain 16-1 via a pressure adjusting safety valve 13-1.

If the control signal of the controller 15 ceases to exist, the spool 12 a of the proportional flow control valve 12 moves backward (to the left) by a spring 12 b. In this case, the piston 11 c is lowerd due to the upper side pressure P2 in the hydraulic cylinder 11. Accordingly, the working fluid ejecting from the lower port 11 a passes through the proportional flow control valve 12 via the working fluid supplying line 18 to be discharged to the drain 16-1.

The controller 15 detects elevating position of the press slide 1 through a slide position detector 20 so that it controls the proportional flow control valve 12 to drive the press die 2 to elevate upwardly and downwardly. Also, the controller 15 receives feedback of the elevating position of the pistion 11 c, that is, the elevating position of the press die 2 through a die cushion position detector 21.

The control signal of the controller 15 is displayed by a command display device 22, for example a computer. At the same time, the command display device 22 may also save data of the control signal, monitor the control signal and etc.

Also, the control signal may also be printed by an output device 23, for example a printer.

FIG. 2 shows graphs of position control and load control of the press die 2 in the die cushion system for press machine 10 as described above. In the figure, graph A represents a position of the press slide 1 in relation to the time and graph B represents a position of the press die which is controlled by the controller 15 and the proportional flow control valve 12 in relation to the time.

Graph C represents a load that the press die 2 receives in a section where reacting force of the press die 2 onto the press slide 1 is exerted to multi levels when the press slide 1 and the press die 2 are in contact therebetween.

Referring to FIG. 2, in an “approaching section” where the press slide 1 starts moving down, the position of the press die 2 is maintained as it is. The press slide 1 and the press die 2 contact therebetween in a “forming section” where press forming of a metal sheet is performed.

During the press forming, the controller 15 controls the proportional flow control valve 12 in multi levels forwardly and backwardly to control the load of the press die 2 to be of multi-level as shown by the graph C. Accordingly, the press die 2 exerts a multi-level reacting force onto the press slide 1 which it is in contact with. In FIG. 2, high load is generated in three levels including an initial load generating section C1 where the press slide 1 is lowered and starts contacting with the press die 2, a maximum load generating section C3 where a maximum load greater than the initial load is generated, and a final load generating section C5 where a final load smaller than the initial load is generated.

In this case, a first interim load generating section C2 where a first interim load smaller than the initial load is generated is interposed between the initial load generating section and the maximum load generating section. And, a second interim load generating section C4 where a second interim load smaller than the final load is generated is interposed between the maximum load generating section and the final load generating section.

Then, plastic deformation of the metal plate is made through five stages. Also, elastic restoration and residual stress which may be generated during the plastic deformation are removed in the sections where multi-level reacting force is generated such as the maximum load generating section C3 and the final load generating section C4.

As the “forming section” belongs to a comparably short period of time, an additional lowering drive source onto the hydraulic cylinder 11 such as the constant pressure supplying means 13, 14 should accompany so that the press die 2 can exert the multi-level reacting force during C1 to C5 sections.

To relieve impact D at the time the press slide 1 and the press die 2 start contacting therebetween in FIG. 2, a pre-acceleration section E may be set up where the hydraulic cylinder 11 is lowered in a low speed at the same time as shown in FIG. 3. Therefore, the impact D′ can be considerably relieved when the press slide 1 and the press die 2 start to contact therebetween.

The technique to instantaneously generate the pre-acceleration section E before “forming section” can not be possible until the constant pressure supplying means 13, 14 as well as the proportional flow control valve 12 are provided to realize the instantaneous forward and backward drive on the hydraulic cylinder 1.

FIG. 4 shows another forming process. In this process, the press slide 1 represented by graph A is lowered and contacts the press die 2 represented by graph B to make a first press forming. Then, the press slide 1 is raised and lowered to contact the press die 2 again to make a second press forming. Then, the press slide 1 is raised and lowered again to contact the press die 2 to make a third press forming.

In this case, the press die 2 receives 0 (zero) load as it is separated from the press slide 1 while the press slide 1 is being raised and lowered on two separate occasions in the “forming section”.

Also, according to the die cushion system for press machine 10 of the present invention, when the press slide 1 represented by the graph A is raised and lowered with predetermined strokes in the “forming section”, the press die 2 represented by the graph B can exert different loads, that is, different reacting forces between the corresponding strokes as shown in FIG. 5 and FIG. 6.

Although the die cushion system for press machine 10 according to the exemplary embodiment of the present invention has been disclosed, various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention.

Therefore, the above embodiment must be regarded as one example provided for description of the present invention, rather than to limit the present invention. 

1. A die cushion system for press machine comprising: a hydraulic cylinder supporting and driving a press die disposed at a lower side in relation to a press slide driven to be raised and lowered at an upper side; a proportional flow control valve receiving working fluid from a tank via a first pump and supplying it to a lower port of the hydraulic cylinder to raise the hydraulic cylinder, and receiving the working fluid within the hydraulic cylinder from the lower port and discharging it to the tank while the hydraulic cylinder is being lowered; a constant pressure supplying means receiving pressured fluid from the tank via a second pump and supplying it to an upper port of the hydraulic cylinder to lower the hydraulic cylinder with the pressured fluid; a slide position detector for detecting position of the press slide; a die cushion position detector for detecting position of the press die; and a controller controlling operation of the proportional flow control valve to adjust lowering and raising of the press die according to the position of the press slide through the slide position detector and the die cushion position detector, and during the lowering process of the press slide, the controller controlling the operation of the proportional flow control valve so that the press die can exert multi-level reacting force onto the press slide.
 2. The die cushion system for press machine according to claim 1 wherein the multi-level reacting force is controlled to generate an initial load at the time the press slide is lowered and starts contacting the press die, a maximum load greater than the initial load and a final load smaller than the initial load.
 3. The die cushion system for press machine according to claim 2 wherein a first interim load smaller than the initial load is generated between the initial load and the maximum load, and a second interim load smaller than the final load is generated between the maximum load and the final load.
 4. The die cushion system for press machine according to claim 3 wherein at least one of the first interim load and the second interim load is 0 (zero) load.
 5. The die cushion system for press machine according to claim 4 wherein the press slide and the press die are separated from each other during the 0 (zero) load.
 6. The die cushion system for press machine according to claim 1 wherein the controller controls the operation of the proportional flow control valve for the press die to be lowered at a speed lower than that of the press slide so as to relieve contacting impact at the time the press slide is lowered and start to contact the press die. 